1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly to an improvement of the so called SOS (Silicon on Sapphire) type semiconductor devices, in which active elements are formed in a single-crystal semiconductor film on an insulating single-crystal substrate.
2. Description of the Prior Art
An SOS type semiconductor device is manufactured by epitaxially forming a silicon single-crystal film on an insulating single-crystal substrate, such as sapphire and spinel, and by forming active elements in the silicon film. In the SOS type semiconductor device, parasitic capacitance (i.e. stray capacitance) can be sharply decreased, so that the operational speed of an integrated circuit of the SOS type semiconductor device is remarkably increased. Generally the silicon film on the insulating substrate is selectively etched to form the so called silicon islands to be used as active elements, which are insulated from each other. Then, the active elements are connected to each other with metal conductive lines. However, in this case there are steps formed between the surfaces of the silicon islands and the surface of the insulating substrate, and as a result, it is difficult to perform micro-circuit fabrication because the metal conductive lines sometimes break at the steps. Therefore, as a means of solving the above mentioned problem of breaking of metal lines, it has been already proposed to use a process involving the selective oxidation of silicon.
According to the above mentioned proposed process, a region between the active elements (i.e. a so-called field region) of the silicon film is etched, but the region of the silicon film is not completely etched, and the remaining silicon film is thermally oxidized to form a silicon dioxide film. As a result, islands of silicon are surrounded by the thusly formed silicon dioxide film and the surfaces of the silicon islands and silicon dioxide film are in the same plane. Such processes have been described in publications, for example, by Alan Capell et al, Process Refinements Bring C-Mos on Sapphire into Commercial Use, Electronics, May (1977) pages 99 to 105.
The SOS type semiconductor device has an advantage of a smaller parasitic capacitance than that of a non-SOS type semiconductor device. However, experiments conducted by the inventor of the present invention have proved that leakage current in the SOS type semiconductor device treated with the above-mentioned selective oxidation process is large. It is considered that this drawback is caused by:
(a) a misfit of the crystal lattices at the interface between the grown silicon film and the sapphire substrate, and
(b) autodoping, i.e. the diffusion of aluminum from the substrate into the grown silicon.